POGIL EXERCISE 13 All about Oxidation States, Oxidation and Reduction

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1 +5 MIN RUN TIME: 85 MIN POGIL 13: Page 1 of 7 POGIL EXERCISE 13 All about Oxidation States, Oxidation and Reduction Each member should locate his/her role in Table 1 and assume his or her role at this time. The new manager takes charge of the POGIL folder and hands out the GRF and RRF to the appropriate members. The new recorder should record the names of the group members on the new GRF and the Reflector should record the start time on her/his form. Table 1. Group Member Role Assignments GROUP TYPE -> GROUPS OF THREE GROUPS OF FOUR MEMBER NO. -> Manager + + Reporter + + Recorder + + Reflector + + Technician Encourager + + SFUC + * * + OBSERVATION I: You know that a reaction has taken place when you detect that oxidation and reduction have taken place. Oxidation and reduction always occur simultaneously in a chemical reaction; thus, reactions where these processes occur are often referred to as redox reactions. Detection of redox reactions requires an analytical eye looking at each element in the chemical equation because only elements can be oxidized or reduced. 1. Can water be oxidized or reduced? Explain your answer: 2. Can carbon in carbon dioxide be oxidized or reduced? Explain your answer: 3. Can O2 be oxidized or reduced? Explain: 4. Can gasoline be oxidized or reduced? Explain: 5. Can sodium be oxidized ore reduced? Explain: Present your responses to the instructor for validation

2 RUN TIME: 85 MIN POGIL 13: Page 2 of 7 OBSERVATION II: Table 2 summarizes the various definitions of oxidation and reduction remember that each criterion refers to elements only. The various definitions are not mutually exclusive; that is, it is common that two, even all definitions can be seen in any given reaction but only one of the three is sufficient to identify the redox process. In subsequent sections we will consider each of the three criteria. Table 2. Operational Definitions of Oxidation and Reduction AN ELEMENT IS OXIDIZED IF It becomes more positive It gains oxygen It loses hydrogen AN ELEMENT IS REDUCED IF It becomes more negative It loses oxygen It gains hydrogen However, before we explore these rules, we need to understand the concept of the oxidation state (or oxidation number) of an element as it exists in each component of the reaction. Oxidation state is a chemist s electron accounting system consisting of the number of pluses and minuses that indicates the relative electronegativity of the elements involved. Like all accounting, the value of the oxidation states are assigned and balanced by specific rules. Although your text has a list of similar rules, the rules listed in Table 3 are the ones used in this course and should be committed to memory. Table 3. Rules for Calculating Oxidation States of Elements 1. The oxidation state of any pure substance (element, molecule, or compound) is The oxidation state of a monatomic ion is the same as its charge. For main group cations, this is the same as the group number; for polyvalent cations the charge is usually calculated using Rule 1; for main group anions the charge is the group number of the element minus The oxidation state of oxygen is -2 unless it exists as the peroxide ion (O2-2 ). 4. The oxidation state of chemically bound hydrogen is +1 unless it is in combination with an alkaline earth metal where it is -1. (e.g. NaH) 5. The oxidation state of a polyatomic ion is its charge. 6. The sum of the oxidation states of a pure substance is For polyatomic ions the sum of elemental oxidation states must equal its charge. As a group fill in the numbered empty cells (Items 6-31) of Table 4 using Observation II and the rules of Table 3. Validate your answers with the instructor. +10 MIN

3 RUN TIME: 85 MIN POGIL 13: Page 3 of 7 Table 4. Oxidation states of Elements in Specific Combinations Chemical Oxidation State Chemical Oxidation State Species OF VALUE Species OF VALUE SO3-2 S +4 CO C +2 Cl - Cl 6 NO2 - N 19 7 Cl 0 N2O5 N 20 ClO - Cl 8 Fe2O3 Fe 21 9 Cl +3 HPO4-2 P 22 ClO3 - Cl 10 O2 O 23 ClO4 - Cl 11 KMnO4 Mn 24 NH CO3-2 C 25 PO4-3 P 13 Na2SO4 S 26 IO Na2SO3 Na 27 H2S H 15 (NH4)2S N P -3 (NH4)2S S 29 NO Cu2SO4 Cu 30 Cu2SO4 S 18 K2CrO7 Cr 31 OBSERVATION III: The first definition of oxidation and reduction involves a change in an element s oxidation state: (1) If an element becomes more positive during a reaction, it has been oxidized; (2) if an element becomes more negative, it has been reduced. Consider the reaction outlined in Equation EQ1: Na + Cl2 Na + + Cl - Equation 2 is the half reaction for sodium indicating that sodium becomes more positive (0 to +1) as the reaction goes to completion by losing one electron. Since sodium has become more positive, it has been oxidized EQ2: Na Na + e - Equation 3 shows the half reaction for chlorine and indicates that each chlorine in the molecule must gain an electron to become a chloride ion. 0-1 EQ3: Cl2 + 2 e - 2 Cl MIN

4 RUN TIME: 85 MIN POGIL 13: Page 4 of 7 Since chlorine goes from 0 to -1, it has been reduced. Notice that the reduction of chlorine requires two electrons one want work! Therefore, two sodiums are required for the reaction to be complete. Finally, there are two more terms that need to be defined related to this topic: reducing agent and oxidizing agent. The pure substance that contains the element that is reduced is called the oxidizing agent; the pure substance that contains the element that is oxidized is called the reducing agent. Remember that neither oxidation or reduction can take place alone they must occur together in the same reaction. Oh, yes, and remember this: all pure substances have an oxidation state of zero. As a group discuss and complete the following reinforcement questions covering Observation: 32. Identify all matter that are described by the term, pure substance. 33. What is a redox half equation? 34. What is the reducing agent in the reaction described by Equation 3? 35. When an element loses an electron, it becomes more and therefor it has been. 36. When an element loses hydrogen, it has been. 37. What is the oxidizing agent in the reaction described by Equation 3? Using all previous information and Figure 1, process Items on the next page as follows: (1) Circle the element you believe has been oxidized. (2) Put a box around the symbol of the element that is reduced. (3-4) Identify the oxidizing and reducing agent. (5-6) write the redox half equations for the oxidation and reduction shown in the reaction and (7) balance the equation. Please submit work to instructor before continuing with Item MIN

5 RUN TIME: 85 MIN POGIL 13: Page 5 of 7 3. reducing 4. oxidizing agent agent Oxidation Half Reaction: Fe 0 Fe +3-3e 6. Reduction Half Reaction: O2 0 2 O e 7a. Oxidation Half Reaction: 4 Fe 0 4 Fe +3-12e 7b. Reduction Half Reaction: 3 O2 0 3 O e 7c. Balanced Equation: 4 Fe + 3 O2 2 Fe2O3 Figure 1. Example of how to process Items K2CrO7 + FeCl2 + HCl FeCl3 + CrCl3 + KCl + H2O 39. MnO2 + HCl Cl2 + H2O + MnCl2 40. Cu + AgNO3 Cu(NO3)2 + Ag 41. NaI + H2SO4 I2 + H2S + Na2SO4 + H2O +70 MIN

6 RUN TIME: 85 MIN POGIL 13: Page 6 of 7 OBSERVATION IV: We will consider the remaining two definitions listed in Table 2 together in this section. An element has been oxidized when it gains oxygen or loses hydrogen; an element has been reduced if it loses oxygen and gains hydrogen. Consider Equation 4 below. EQ4: CH4 + O2 CO2 + 2 H2O As you can see, carbon gains oxygen and loses hydrogen and is therefore oxidized. This in turn means that methane (CH4) must be the reducing agent (the hydrogen donor). What is harder to see is the element that that loses oxygen. Since the oxygens are bound to each other in molecular oxygen (O2), the splitting apart of the two atoms means that each of the oxygen atoms has lost an oxygen. Atomic oxygen has been reduced making oxygen the oxidizing agent. Use the information in Observation IV to process Items Items refer to Equation 5. EQ5: C2H6 + O2 CO2 + H2O 42. Which element has gained hydrogen? 43. Which element has lost hydrogen? 44. Which element has gained oxygen? 45. Which element has lost oxygen? 46. Which element has been oxidized? 47. Which element has been reduced? 48. What is the oxidizing agent? 49. What is the reducing agent? Recorder should present responses for Items to the instructor for validation. +75 MIN

7 RUN TIME: 85 MIN POGIL 13: Page 7 of 7 Items refer to Equation 6 EQ6: Potassium metal and chlorine gas will react vigorously to produce potassium chloride. 44. Write a balanced chemical equation that would describe this reaction (Equation 7). EQ7: 45. What has been oxidized in this reaction? Give the reason for your answer. 46. What has been reduced in this reaction? Give the reason for your answer. 47. What is the reducing agent in this reaction? Recorder should present responses for Items to the instructor for validation. Items refer to Equation 6 EQ6: CH3COCH3 + O2 CO2 + H2O 48. Which component has been oxidized? Explain: 49. Which component has been reduced? Explain: 50. Balance Equation 6 Present your responses to the instructor for validation. EXERCISE END. Managers should collect the GRF, the Recorders exercise and the RRF, paperclip them together, and place in the left pocket of the folder. The folder should be closed and left on the table. NOTIFY INSTRUCTOR WHEN FINISHED. WAIT FOR INSTRUCTIONS. +85 MIN

POGIL EXERCISE 12 How Can You Predict Chemical Reactions?

RUN TIME: 60 MIN POGIL 12 Page 1 of 7 POGIL EXERCISE 12 How Can You Predict Chemical Reactions? Each member should assume his or her role at this time. The new manager takes charge of the POGIL folder